This invention relates to fluorescent lamps and to methods and materials for the fabrication thereof. More particularly, this invention concerns fluorescent lamp phosphors and coatings for fluorescent lamp phosphors.
Commercially available fluorescent lamps comprise an elongated tubular envelope having a pair of electrodes sealed into the opposite ends thereof. The envelope contains a gaseous atmosphere, which may be a mixture of a rare gas and a metal vapor, such as mercury vapor. The interior surface of the envelope is coated with a finely-divided fluorescent material which is exposed to the electrical discharge between the two electrodes, and is excited by the ultraviolet radiations emitted by this discharge. The fluorescent coating is usually applied by suspending particulate fluorescent or phosphor material in a suitable binder, flushing the interior of the tube with the suspension, permitting the excess suspension to drain out of the envelope, and then firing the interior wall of the coated envelope at a temperature which promotes adherence of the coating to the envelope walls and removes, generally by volatilization, the binder material. There results a phosphor layer or coating adhered to the inside or interior surfaces of the tubular envelope.
Thus, in the course of manufacture, as well as during operation of these fluorescent lamps, the phosphors used in these lamps experience a hostile environment. During manufacture, the lamp is baked at temperatures approximating 600.degree. C. which can cause serious degradation of some phosphors. While the lamp is operating, the phosphor is in a mercury vapor discharge where it is subjected to ultraviolet radiation and bombardment by electrons and mercury atoms and ions. These factors may be responsible for lamp or phosphor maintenance losses; i.e., for the time-dependent decrease in luminous flux found in all fluorescent lamps.
One method for improving brightness and maintenance is to shield the phosphor from these effects by coating the phosphor particles with a protective film, and numerous attempts have been made to protectively coat the particulate phosphor material by using selected additions to the coating suspension. Such additives include silica, boric anhydride, sodium borate, mixtures of barium and ammonium nitrates, crystalline alkali halides, calcium pyrophosphate, ammonium dihydrogen phosphate, orthophosphoric acid, etc.
These materials have several disadvantages as fluorescent lamp phosphor coatings. The polycrystalline additions, by their very nature, cannot be expected to form an impervious film on the phosphors, therefore providing incomplete protection for the phosphor. Some of the materials are not chemically compatible with the phosphor, or are not sufficiently resistant to attack by mercury atoms. Some of the coating materials absorb ultraviolet radiation at 254 nanometers, the primary excitation wavelength for the lamp phosphor. Thus, while providing protection for the phosphor against the hostile lamp environment, such coatings may contribute to undesirable decreases in phosphor brightness. Some glassy coating materials such as boric anhydride, while transparent to ultraviolet radiation in the pure state, become absorptive upon contamination with sodium ion which migrates from the glass envelope to the phosphor during lamp operation.
Prior art phosphor coatings provide protection of the phosphor against the hostile lamp environment to varying extents, but generally are not capable of making a direct contribution to lamp brightness.